Heavier payload caused Indian rocket's failure: Expert

Instability introduced by excessive payload weight was most likely responsible for the failure of an Indian rocket's launch on Christmas day, an expert in the field and former scientist of the Indian Space Research Organization (ISRO) says.

The scientist, with over two decades of experience with rocket motors, who did not want to be quoted has disputed ISRO's reasoning that the Rs 3 billion ($66 million) mission that was meant to launch an advanced communications satellite was brought down because some cables snapped during the ascent of the Geostationary Satellite Launch Vehicle (GSLV).

ISRO chairman K Radhakrishnan said at a post-launch press conference Dec 25 that cables carrying control signals from the on-board computer to the first stage got snapped and, as the uncontrolled vehicle started deviating from its flight path, it had to be destroyed.

Radhakrishnan, however, did not say why the cables snapped. ISRO's routine post-launch press release gave no details but only made a cryptic announcement that the "launch of GSLV-F06/GSAT-5P mission (was) not successful".

While admitting that the cable might have snapped, the unnamed source, however, disputes the claim that snapping of the connector cable to the control system actually triggered the tragedy. "The cable snapping was the effect (of the GSLV breaking) and not the cause of the mishap," he maintained.

"The cable joints cannot snap just like that," the scientist said. He said the connectors are locked so well they cannot snap unless the vehicle itself breaks.

According to the source, the GSLV most likely broke due to instability caused by the heavy payload - heavier than what the rocket had lifted in its previous missions.

At 2,310 kg, the GSAT-5P communication satellite carried in the ill-fated mission was the heaviest payload ever lifted by a GSLV. It was 180 kg heavier than the INSAT-4CR launched successfully by a GSLV in 2007, 400-kg heavier than Edusat launched in 2003 and about 800 kg heavier than GSAT-1 launched in 2001.

According to the source, the following sequence of events might have taken place leading to the Christmas day disaster:

* The excessive weight of the payload made the rocket tilt due to aerodynamic forces.

* The control signals to correct the attitude must have gone to the first stage gymbal system as expected normally.

* But the control system failed to arrest the tilting as it was beyond the limit (plus or minus 4 degrees) of controllability.

* The vehicle broke, as it was not capable of taking the higher structural load brought about by tilting beyond the limit.

The source said ISRO scientists, in simulated tests, must have definitely looked at vehicle stability after increasing the payload "but it is possible that something went wrong".

According to the source, the fact that ISRO had so far never encountered problems with the first two stages of GSLV further strengthens his hypothesis that the excess payload carried by the GSLV triggered the series of events leading to the rocket's premature death.